This invention relates to RF load and source pull testing of medium and high power RF transistors and amplifiers using remotely controlled electro-mechanical impedance slide screw tuners. Modern design of high power RF amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models only.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” (FIG. 1). Load pull is a measurement technique employing microwave impedance tuners (2,4) and other microwave test equipment, such as signal sources (1), test fixtures and DUT (3) and power meters (5), the whole controlled by a computer (6); the computer controlling and communicating with the tuners (2,3) and other equipment (1,3,5) uses digital cables (7,8,9). The tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (see ref. 1); tuners allow determining the optimum impedance conditions for designing amplifiers and other microwave components for specific performance targets, such as gain, efficiency, intermodulation etc.; this document refers hence to “tuners” as being “impedance tuners”, in order to distinct from “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits (see ref. 2).
Impedance tuners consist, in general, of a transmission line (23, 24) and adjustable probes (22), FIG. 2; said probe (22) is attached to a precision vertical axis (21) which is mounted in a mobile carriage (28); said axis (21) can move the probe (22) vertically (216) towards the center conductor (23) and the carriage (28) can move the probe (22) horizontally 217) parallel to said center conductor (23). The vertical movement (216) changes the amplitude of the reflection factor seen at the tuner test port (25) whereas the horizontal movement (217) changes the phase. This way the whole Smith chart is covered allowing quasi-infinity of impedances from Zmin to Zmax to be synthesized at any given frequency within the “tuning range” of said tuner. Typical values of state of the art tuners are |Zmin|=2 Ohm and |Zmax|=1250 Ohm; this corresponds to Voltage Standing Wave Ratio (VSWR) of 25 (FIG. 7). The relation between reflection factor and impedance is given by GAMMA=|GAMMA|*exp(jΦ)=(Z−Zo)/(Z+Zo) {eq. 1}, wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50 Ohm [3]. The equivalent is the Voltage Standing Wave Ratio: VSWR=(1+|GAMMA|)/(1−|GAMMA|) {eq. 2}, see ref. 3.
Metallic probes (22) or slugs are made in a cubical form (41) with a concave bottom (35) which allows to capture, when approaching the center conductor (32) (see ref. 4 and FIG. 11) the electric field which is concentrated in the area (36) between the center conductor (32) and the ground planes of the slabline (31) (FIG. 3). This “field capturing” allows creating high and controllable reflection factors. The critical part is the required proximity and accuracy of both the vertical and horizontal probe movement (FIG. 7), whereby changes in the vertical probe position (72) of a few micrometers affects the VSWR by a large amount.
When microwave power is injected into the tuner some of it is absorbed by the center conductor of the slabline (66). This leads to a rise of its temperature and associated longitudinal expansion (615, 616 in FIG. 6(a)). Since the center conductor (66) is fixed at both ends on the coaxial connectors it has only limited range for expansion; this will lead to “bending” (67) in FIG. 6(a); “Bending” happens in different ways, depending on the pre-disposition of the center conductor, which cannot be “perfectly” straight (FIGS. 6(b2) and 6(b3)). Whereas in an ideal situation (FIG. 6(b1)) the center conductor is positioned exactly in the center of the slabline channel, when the center conductor is heated and buckles (see ref. 6) it may either deflect sidewise (617) or downwards (614); of course it may also deflect upwards (not shown), in which case we may have a short circuit same as when it deflects sidewise (617). In either case the effect is at best loss of accuracy or at worst an electrical short and damage of the tuner and/or the DUT.
In case an electrical short occurs followed by either temporary or permanent damage of the tuner or the DUT, at least the operator will be alerted and can take measures to correct the situation, as shown later on in this disclosure. But if it does not come to a short (case FIG. 6(b3)), then the effect will be false measurement and spurious resonances (53) in the otherwise typically flat (52) response of the reflection factor (S11), which is created when the tuning probe is close to the center conductor as shown in FIG. 6(b1). This is because, as shown in FIG. 7, in the area (72) a relatively small movement between the tuner probe (615) and center conductor (611) in FIG. 6, will change the calibrated VSWR substantially; in other words the data retrieved from the measurement instruments (FIG. 1) will be associated and recorded at the wrong VSWR values. There will be no warning, just wrong data; it comes even worse: should the operator not trust the measured data and develop doubts about the tuner accuracy, and should he disconnect the tuner from the test setup in order to re-calibrate or verify the calibration on a VNA (see FIG. 8), he, probably, will find that the tuner is accurate. This is because, during the dismantling of the setup the center conductor will cool down and recover its initial (calibrated) position (transition from states in FIGS. 6(b2), 6(b3) back to the state of FIG. 6(b1); this can be a very critical systemic problem for high power testing using such tuners. This invention discloses embodiments of a technique allowing the avoidance of such systemic test problems.